i've just installed a whole new bunch of font sum 500/600, and now when i go on web-pages such as google, myspace something is changing the font, like its replacing my default font....
Many webpages specify fonts, but use the default if the specified fonts are not available on your system. When you installed all those fonts, you increased the chances that the fonts specified by the webpage designer are on your system.
As you install more and more fonts FontBook becomes less and less attractive. I use FontExplorer by Linotype (free!) and it works wonders; even cleans the system fonts folder leaving only the required fonts there for better performance: